U.S. patent application number 12/598959 was filed with the patent office on 2010-06-24 for lighting device for display device and display device.
This patent application is currently assigned to SHARP KABUSHIKI KAISHA. Invention is credited to Masahiro Arai.
Application Number | 20100156952 12/598959 |
Document ID | / |
Family ID | 40001889 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100156952 |
Kind Code |
A1 |
Arai; Masahiro |
June 24, 2010 |
LIGHTING DEVICE FOR DISPLAY DEVICE AND DISPLAY DEVICE
Abstract
A lighting device for a display device includes a light source
and a light source control device arranged to control the light
source. The light source control device is arranged to generate a
pulse signal as a light source control signal Vcon to control the
light source. The light source control signal Vcon includes pulses,
which individually have different shapes.
Inventors: |
Arai; Masahiro; (Osaka-shi,
JP) |
Correspondence
Address: |
SHARP KABUSHIKI KAISHA;C/O KEATING & BENNETT, LLP
1800 Alexander Bell Drive, SUITE 200
Reston
VA
20191
US
|
Assignee: |
SHARP KABUSHIKI KAISHA
Osaka-shi, Osaka
JP
|
Family ID: |
40001889 |
Appl. No.: |
12/598959 |
Filed: |
December 25, 2007 |
PCT Filed: |
December 25, 2007 |
PCT NO: |
PCT/JP2007/074794 |
371 Date: |
November 5, 2009 |
Current U.S.
Class: |
345/690 ;
345/102 |
Current CPC
Class: |
G02F 1/133604 20130101;
G02F 2201/505 20130101; H05B 41/3927 20130101; H05B 41/2828
20130101; G09G 3/3406 20130101 |
Class at
Publication: |
345/690 ;
345/102 |
International
Class: |
G09G 3/36 20060101
G09G003/36; G09G 5/10 20060101 G09G005/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 2007 |
JP |
2007-130603 |
Claims
1-8. (canceled)
9. A lighting device for a display device comprising: a light
source; and a light source control device arranged to control said
light source; wherein said light source control device is arranged
to generate a pulse signal as a light source control signal to
control the light source; and said light source control signal
includes pulses, which individually have different shapes.
10. A lighting device for a display device as in claim 9, wherein:
said light source control device includes a periodic signal
generating circuit, an aperiodic signal generating circuit, and an
arithmetic logic circuit arranged to perform a logical operation on
a plurality of signals; and said arithmetic logic circuit is
arranged to generate said light source control signal by performing
a logical operation on a periodic signal generated from said
periodic signal generating circuit and an aperiodic signal
generated from said aperiodic signal generating circuit.
11. A lighting device for a display device as in claim 9, wherein:
said light source control device includes a periodic signal
generating circuit and an arithmetic logic circuit arranged to
perform a logical operation on a plurality of signals; said
periodic signal generating circuit includes at least three periodic
signal generating circuits arranged to generate signals
individually at different frequencies; and said arithmetic logic
circuit is arranged to generate said light source control signal by
performing a logical operation on at least two periodic signals
which are randomly selected for each time period from periodic
signals generated from said periodic signal generating
circuits.
12. A lighting device for a display device as in claim 9, wherein:
a correction circuit is provided between said light source and said
light source control device; and said correction circuit is
arranged to control said light source control signal generated from
said light source control device, so as to provide a constant time
ratio between light-on period and light-off period in one vertical
period.
13. A lighting device for a display device as in claim 10, wherein:
a correction circuit is provided between said light source and said
light source control device; said correction circuit is arranged to
control said light source control signal generated from said light
source control device, so as to provide a constant time ratio
between light-on period and light-off period in one vertical
period; and said correction circuit includes: an input section
arranged to receive, as a reference signal, a signal generated from
said periodic signal generating circuit; a detection section
arranged to detect, as a phase difference, a difference between
said reference signal and said light source control signal in ratio
of light-on period in one vertical period; and an output section
arranged to output a corrected light source control signal as a
result of correcting said light source control signal by said phase
difference.
14. A lighting device for a display device as in claim 12, wherein
said correction circuit includes: an input section arranged to
receive, as a reference signal, a previous light source control
signal that has been generated as said light source control signal;
a detection section arranged to detect, as a phase difference, a
difference between said reference signal and a subsequent light
source control signal in a ratio of light-on period in one vertical
period; and an output section arranged to output a corrected light
source control signal as a result of correcting said subsequent
light source control signal by said phase difference.
15. A display device comprising: a lighting device for a display
device according to claim 9; and a display panel arranged to
perform display by use of light from said lighting device for a
display device.
16. A display device as in claim 15, wherein said display panel is
a liquid crystal panel including liquid crystal material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a lighting device for a
display device and a display device using the same.
[0003] 2. Description of the Related Art
[0004] In a display device having non-luminous optical elements as
typified by a liquid crystal display device, a backlight device is
provided on the backside of a display panel such as a liquid
crystal panel, so as to illuminate the display panel.
[0005] Conventionally, a voltage-controlled dimming system or a PWM
(Pulse Wide Modulation) dimming system is used as a dimming system
for a backlight. The voltage-controlled dimming system controls the
voltage applied to a fluorescent tube via an inverter for dimming
purposes, by which a wide dimming range cannot be provided.
[0006] On the other hand, the PWM dimming system periodically turns
a light source on and off for dimming purposes, while controlling
the time ratio between light-on periods and light-off periods. This
system can provide a wide dimming range by adjusting the on-off
time cycle. Therefore, many liquid crystal display devices employ
the PWM dimming system for a backlight.
[0007] In recent years, techniques for using a plurality of PWM
signals in combination have been also developed. For example, in
order to improve visibility of moving images, JP-A-2006-53520
discloses a technique that generates a new periodic signal by
combining two kinds of PWM signals, i.e., a PWM signal for setting
light-on periods and a PWM signal for setting light-off or black
insertion periods, by which an on-off operation of a light source
is repeated during each light-on period.
[0008] In a backlight device thus using the PWM dimming system for
dimming control, beat tones may be generated during the dimming
control of fluorescent tubes, due to the second and third harmonics
of a dimming control frequency. The beat tones may cause discomfort
for a user, and therefore generation thereof must be
suppressed.
[0009] The beat tones may result from various factors. The factors
include an abrupt increase of a tube current flowing through a
fluorescent tube and a momentary flow of an excitation current
immediately preceding turn-on, which cause magnetostriction (called
a whisker voltage), known to contribute to generating beat tones
(as shown in JP-A-2004-55431, for example), when the fluorescent
tube turns from OFF to ON.
[0010] According to the invention described in JP-A-2004-55431,
elimination of beat tones can be achieved by a dimming means that
controls the pulse rise time of PWM signals in order to suppress
the whisker voltage.
[0011] However, the control of the pulse rise time of PWM signals
fails to achieve adequate elimination of beat tones. Therefore, the
factors contributing to generating beat tones may include factors
other than the whisker voltage, and the inventor of the present
application discovered that periodical vibration of a chassis
covering light sources contributes to generating beat tones.
SUMMARY OF THE INVENTION
[0012] Accordingly, preferred embodiments of the present invention
provide a lighting device for a display device having a
construction capable of preventing periodical vibration of a
chassis and thereby preventing or minimizing beat tones. In
addition, preferred embodiments of the present invention provide a
high-quality and highly-reliable display device including such an
improved lighting device.
[0013] A lighting device for a display device, according to a
preferred embodiment of the present invention, includes alight
source and a light source control device configured to control the
light source, in which the light source control device is arranged
to generate a pulse signal as a light source control signal to
control the light source. The light source control signal includes
pulses, which individually have different shapes.
[0014] The inventor of the present application has thoroughly
examined the beat tones, and discovered that vibration of a chassis
caused by leakage current from a fluorescent tube (or light source)
is a major factor in generating the beat tones. It has also been
discovered that the generation of beat tones is facilitated by the
monotonous periodicity of a signal (hereinafter, sometimes referred
to as a periodic signal) generated by a periodic signal generating
circuit such as a PWM signal generating circuit. In this regard,
the inventor of the present application has developed the following
causal theory.
[0015] The chassis is typically formed of conductive metal.
Therefore, when current is applied to the light source, a capacitor
is formed between the light source and the chassis, which may cause
current leakage from the light source to the chassis. A force
acting on the chassis can be generated due to the leakage current,
which causes the chassis to vibrate resulting in beat tones.
[0016] Further, as a result of on-off control according to a
periodic signal or pulse signals generated at a constant frequency,
the light source periodically repeats ON and OFF, and accordingly
the above-described leakage current causes periodic vibration of
the chassis. When the chassis thus vibrates constantly with the
same period, the vibration amplitude of the chassis is increased,
and thereby the generation of beat tones due to the vibration of
the chassis is facilitated. Moreover, the beat tones thus generated
are within the audible frequency range, and are also periodic.
Therefore, a user will be forced to hear the same noise constantly
and repeatedly, and thereby the user will feel more
uncomfortable.
[0017] In view of this, the lighting device for a display device,
according to a preferred embodiment of the present invention,
provides a light source control signal to control the light source,
which includes pulses individually having different shapes.
Thereby, beat tones can be prevented or minimized.
[0018] That is, if a light source control signal formed of pulses
individually having different shapes is thus used for control of
the light source, the light source can be aperiodically turned
on.
[0019] Therefore, the period of vibration of the chassis caused by
leakage current from the light source can be non-constant,
resulting in prevention of an increase in vibration amplitude of
the chassis. Consequently, the facilitation of generation of beat
tones, which may be due to periodic vibration of the chassis, can
be prevented.
[0020] Other features, elements, steps, characteristics and
advantages of the present invention will become more apparent from
the following detailed description of preferred embodiments of the
present invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is an exploded perspective view showing the general
construction of a liquid crystal display device according to
preferred embodiment 1 of the present invention.
[0022] FIG. 2 is a sectional view showing the general construction
of the liquid crystal display device shown in FIG. 1.
[0023] FIG. 3 is a block diagram showing a circuit configuration
involved in light source control of a backlight device included in
the liquid crystal display device shown in FIG. 1.
[0024] FIG. 4 is a waveform chart showing signals outputted from
various sections of the backlight device included in the liquid
crystal display device shown in FIG. 1.
[0025] FIG. 5 is a block diagram showing a circuit configuration
involved in light source control of a backlight device included in
a liquid crystal display device according to preferred embodiment 2
of the present invention.
[0026] FIG. 6 is a waveform chart showing signals outputted from
various sections of the backlight device shown in FIG. 5.
[0027] FIG. 7 is a block diagram showing a circuit configuration
involved in light source control of a backlight device included in
a liquid crystal display device according to preferred embodiment 3
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred Embodiment 1
[0028] A preferred embodiment 1 of the present invention will be
explained with reference to FIGS. 1 to 4.
[0029] FIG. 1 is an exploded perspective view showing the general
construction of a liquid crystal display device according to the
present preferred embodiment. FIG. 2 is a sectional view showing
the general construction of the liquid crystal display device. FIG.
3 is a block diagram showing a circuit configuration involved in
light source control of a backlight device. FIG. 4 is a waveform
chart showing waveforms outputted from various sections of the
backlight device.
[0030] The general construction of the liquid crystal display
device (display device) 10 according to the present preferred
embodiment will be explained first. Referring to FIGS. 1 and 2, the
liquid crystal display device 10 preferably includes a liquid
crystal panel 11 having a rectangular or substantially rectangular
shape, and a backlight device (lighting device for a display
device) 12 as an external light source, which are integrally held
by a bezel 13 and the like. The liquid crystal panel 11 includes a
pair of glass substrates, which are attached to each other so as to
face each other while a gap of a predetermined size is kept
therebetween. Liquid crystal is sealed between the glass
substrates. On one of the glass substrates, components such as
switching elements (e.g., TFTs) connected to source wiring lines
and gate wiring lines running at right angles to each other, and
pixel electrodes connected to the switching elements are provided.
On the other of the glass substrates, components such as a counter
electrode and a color filter having R, G, and B color sections
arranged in a predetermined pattern are provided.
[0031] Next, the backlight device 12 will be explained. The
backlight device 12 is a so-called direct-light type backlight
device that includes a plurality of linear light sources (e.g.,
cold cathode tubes (light sources) 17 as high-pressure discharge
tubes, in the present preferred embodiment), which are positioned
directly below the back surface of the liquid crystal panel 11
(i.e., the panel surface on the opposite side of the display side),
and are arranged along the panel surface.
[0032] The backlight device 12 includes a backlight chassis
(sometimes referred to as a chassis) 14 having a substantially
box-like shape with an opening on its upper side, and a plurality
of optical members 15 (e.g., a diffuser plate, a diffusing sheet, a
lens sheet and an optical sheet, in order from the lower side of
the figure) which are arranged to cover the opening of the
backlight chassis 14. Further included is a frame 16 arranged to
hold the optical members 15 on the backlight chassis 14. The
backlight chassis 14 contains the cold cathode tubes 17, rubber
holders 18 arranged to hold the end portions of the cold cathode
tubes 17, lamp holders 19 arranged to collectively cover the cold
cathode tubes 17 and the holders 18, and lamp clips 20 arranged to
mount and hold the cold cathode tubes 17 on the backlight chassis
14. Note that the optical member 15 side of the cold cathode tubes
17 corresponds to the light emitting side of the backlight device
12.
[0033] Each cold cathode tube 17 preferably has an elongated
tubular shape, for example. A number (e.g., eighteen in FIG. 1) of
cold cathode tubes 17 are arranged in the backlight chassis 14 so
that the longitudinal direction (or axial direction) thereof
corresponds with the long-side direction of the backlight chassis
14. On the other hand, the lamp clips 20, arranged to mount the
cold cathode tubes 17 to the backlight chassis 14, function as clip
members for holding light sources, and are preferably made of
synthetic resin (e.g., polycarbonate). The plurality of lamp clips
20 are mounted on the backlight chassis 14 so as to support each of
the cold cathode tubes 17 preferably at two or three points spaced
along the longitudinal direction thereof.
[0034] The backlight chassis 14 is preferably formed of a metallic
plate. A light reflecting sheet 14a is provided on the inner
surface side (light source side) of the backlight chassis 14, which
defines a light reflecting surface. The backlight chassis 14 thus
includes the light reflecting sheet 14a, and thereby the light from
the cold cathode tubes 17 can be reflected to the optical members
15 such as the diffuser plate (hereinafter, sometimes referred to
as "the diffuser plate 15 and the like"). The light reflecting
sheet 14a can be formed of a resin sheet having light reflectivity,
for example.
[0035] An inverter board 30 to supply drive voltage to the cold
cathode tubes 17 is mounted to the backlight chassis 14, or
specifically, mounted on the opposite side of the cold cathode
tubes 17 (i.e., on the opposite side of the light emitting
surface). The inverter board 30 includes an inverter unit 40
arranged to drive the cold cathode tubes 17 and a light source
driving circuit (or light source control device) 50 arranged to
generate a signal to be used for control of the inverter unit 40.
Further, a PLL (Phase Locked Loop) 60 as a correction circuit
arranged to correct a signal generated from the light source
driving circuit 50 is provided between the inverter unit 40 and the
light source driving circuit 50.
[0036] Hereinafter, the general configuration of the light source
driving circuit 50 and the signals to be generated from respective
circuits will be explained with reference to FIGS. 3 and 4,
respectively. The light source driving circuit 50 includes a PWM
signal generating circuit (periodic signal generating circuit) 51,
a current-controlled dimming signal generating circuit (aperiodic
signal generating circuit) 52, and a mixing circuit (arithmetic
logic circuit) 53. In order to regularly repeat on-off operation of
the light sources, the PWM signal generating circuit 51 generates a
PWM signal (periodic signal) Vpwm as a periodic pulse signal, which
is synchronized with a vertical synchronizing signal generated from
a video signal processing circuit (not shown). On the other hand,
the current-controlled dimming signal generating circuit 52
generates a current-controlled dimming signal (aperiodic signal)
Vant as an aperiodic pulse signal. The current-controlled dimming
signal Vant includes the same number of pulses within one vertical
period, as that of the PWM signal.
[0037] The PWM signal Vpwm and the current-controlled dimming
signal Vant are individually inputted to the mixing circuit 53,
which is connected to the PWM signal generating circuit 51 and the
current-controlled dimming signal generating circuit 52. In the
mixing circuit 53, a logical operation is performed on respective
pulses of the signals. Thereby, the mixing circuit 53 superimposes
the pulses of the aperiodic current-controlled dimming signal Vant
on the pulses of the periodic PWM signal Vpwm, and thereby
generates a light source control signal Vcon that includes pulses
individually having different shapes, as shown in FIG. 4.
[0038] The PLL (or correction circuit) 60 generates a corrected
light source control signal Vout as a result of correction for the
light source control signal Vcon, which is performed so that the
ratio of light-on periods in one vertical period coincides with the
corresponding ratio provided by a PWM signal Vpwm as a reference
signal, as follows.
[0039] The PLL 60 includes a reference signal generating unit
(input section) 61, a phase comparator (detection section) 62, and
a signal processing unit (output section) 63. The reference signal
generating unit 61 receives a PWM signal Vpwm from the PWM signal
generating circuit 51, and synchronizes the PWM signal Vpwm with a
vertical synchronizing signal generated from the video signal
processing circuit (not shown). The resultant signal is outputted
to the phase comparator 62, as a reference signal to be used as a
reference for correction.
[0040] The phase comparator 62 receives the light source control
signal Vcon generated from the mixing circuit 53, and compares the
waveform of the light source control signal Vcon with that of the
above reference signal (or PWM signal Vpwm). The difference in
ratio of light-on periods in one vertical period is detected as a
phase difference, and an error component indicating the light-on
period difference between the light source control signal Vcon and
the reference signal is outputted to the signal processing unit
63.
[0041] The signal processing unit 63 generates, based on the
received error component, a correction signal that is to be used
for correcting the light source control signal Vcon so that the
ratio of light-on periods in one vertical period coincides with
that provided by the reference signal. The correction signal is
outputted to the phase comparator 62 described above.
[0042] The phase comparator 62 superimposes the correction signal
on the light source control signal Vcon, so that the ratio of
light-on periods in one vertical period, provided by the light
source control signal Vcon, approaches that provided by the
reference signal. The detection of a phase difference by the phase
comparator 62, the generation of a correction signal by the signal
processing unit 63, and the correction of the light source control
signal Vcon by the phase comparator 62 are iteratively performed.
Consequently, correction for the light source control signal Vcon,
originally having been inputted to the phase comparator 62, can be
achieved so that the ratio of light-on periods in one vertical
period coincides with the corresponding ratio provided by the
reference signal. Thus, the signal is converted into a corrected
light source control signal Vout capable of providing a constant
ratio of light-on periods in one vertical period, and is outputted
to the inverter unit 40, via the signal processing unit 63.
[0043] The inverter unit 40 applies a voltage based on the received
corrected light source control signal Vout to the cold cathode
tubes 17, so as to drive the cold cathode tubes 17.
[0044] As described above, according to the present preferred
embodiment, the mixing circuit (arithmetic logic circuit) 53
performs a logical operation on a periodic PWM signal (periodic
signal) Vpwm generated by the PWM signal generating circuit
(periodic signal generating circuit) 51 and an aperiodic
current-controlled dimming signal (aperiodic signal) Vant generated
by the current-controlled dimming signal generating circuit
(aperiodic signal generating circuit) 52, so as to generate a light
source control signal Vcon that can be an aperiodic signal
including pulses individually having different shapes. The light
sources are controlled according to pulse shapes of the light
source control signal Vcon, and therefore can be aperiodically
turned on. Accordingly, the period of vibration of the backlight
chassis 14 caused by leakage current from the light sources can be
non-constant, resulting in prevention of increase in vibration
amplitude of the backlight chassis 14. Consequently, the
facilitation of generation of beat tones, which may be due to
periodic vibration of the backlight chassis 14, can be
prevented.
[0045] Further, the PLL (correction circuit) 60 corrects the light
source control signal Vcon, so as to output a corrected light
source control signal Vout capable of providing a constant time
ratio between light-on periods and light-off periods in one
vertical period. Specifically, the PLL 60 corrects the light source
control signal Vcon using a PWM signal Vpwm as a reference signal
generated by the PWM signal generating circuit 51, so that the
ratio of light-on periods in one vertical period coincides with
that provided by the reference signal. Thereby, a corrected light
source control signal Vout capable of providing the ratio of
light-on periods in one vertical period, being coincident with that
provided by the PWM signal Vpwm, can be outputted. According to
this construction, the periods of light-on of the light sources, in
one vertical period as a reference period for one frame of images
to be displayed, can be constant in time length, regardless of
respective pulse shapes of the light source control signal Vcon.
Consequently, the light sources can provide stable brightness,
which enables provision of a high-quality display device.
Preferred Embodiment 2
[0046] Next, a preferred embodiment 2 of the present invention will
be explained with reference to FIGS. 5 and 6. The difference from
the above preferred embodiment 1 is that a light source control
device preferably includes three PWM signal generating circuits,
instead of including a PWM signal generating circuit and a
current-controlled dimming signal generating circuit. The other
constructions are preferably similar to the above preferred
embodiment. Therefore, the same elements as the above preferred
embodiment are designated by the same symbols, and redundant
explanations are omitted.
[0047] FIG. 5 is a block diagram showing a circuit configuration
involved in light source control of a backlight device included in
a liquid crystal display device according to the present preferred
embodiment. FIG. 6 is a waveform chart showing signals outputted
from various sections of the backlight device.
[0048] A light source driving circuit 70 preferably includes three
PWM signal generating circuits 71A, 71B, 71C and a mixing circuit
72. The PWM signal generating circuits 71A, 71B, 71C generate pulse
signals as PWM signals VpwmA, VpwmB, VpwmC at different
frequencies, respectively, and outputs the signals to the mixing
circuit 72.
[0049] The mixing circuit 72 includes a selector 73 and an
arithmetic unit 74. The selector 73 randomly selects two kinds of
PWM signals VpwmX, VpwmY from three kinds of PWM signals VpwmA,
VpwmB, VpwmC for each time period, and outputs the selected
signals. The arithmetic unit 74 performs a logical operation on
respective pulses of the selected PWM signals VpwmX, VpwmY, and
thereby generates an aperiodic light source control signal Vcon
that includes pulses individually having different shapes. Thus,
the mixing circuit 72 can generate an aperiodic light source
control signal Vcon by the logical operation using the periodic PWM
signals VpwmA, VpwmB, VpwmC.
[0050] The PLL 60 corrects the aperiodic light source control
signal Vcon outputted from the mixing circuit 72, so that the ratio
of light-on periods in one vertical period coincides with the
corresponding ratio provided by a PWM signal VpwmA as a reference
signal. Thereby, a corrected light source control signal Vout is
generated. The corrected light source control signal Vout is
outputted to the inverter unit 40. The inverter unit 40 applies a
voltage based on the corrected light source control signal Vout to
the cold cathode tubes 17, so as to drive the cold cathode tubes
17.
[0051] As described above, according to the present preferred
embodiment, three PWM signal generating circuits 71A, 71B, 71C
generate PWM signals VpwmA, VpwmB, VpwmC at different frequencies,
respectively. Further, the mixing circuit 72 randomly selects two
kinds of signals from the three kinds of PWM signals VpwmA, VpwmB,
Vpwmc for each time period, and performs a logical operation on the
signals. Thereby, an aperiodic light source control signal Vcon
including pulses individually having different shapes can be
outputted. The light sources are controlled according to pulse
shapes of the light source control signal Vcon, and therefore can
be aperiodically turned on. Accordingly, the period of vibration of
the backlight chassis 14 caused by leakage current from the light
sources can be non-constant, resulting in prevention of increase in
vibration amplitude of the backlight chassis 14. Consequently, the
facilitation of generation of beat tones, which may be due to
periodic vibration of the backlight chassis 14, can be
prevented.
Preferred Embodiment 3
[0052] A preferred embodiment 3 of the present invention will be
explained with reference to FIG. 7. The difference from the above
preferred embodiments 1 and 2 is that a previously-generated light
source control signal is used as a reference signal, instead of
using a PWM signal as a reference signal. The other constructions
are preferably similar to the above preferred embodiments.
Therefore, the same elements as the above preferred embodiments are
designated by the same symbols, and redundant explanations are
omitted.
[0053] FIG. 7 is a block diagram showing a circuit configuration
involved in light source control of a backlight device included in
a liquid crystal display device according to the present preferred
embodiment.
[0054] A PLL 80 includes a reference signal generating unit 81, a
phase comparator 82, and a signal processing unit 83. The reference
signal generating unit 81 receives a corrected light source control
signal VoutA that has been previously corrected. The signal is
synchronized with a vertical synchronizing signal generated from a
video signal processing circuit (not shown), and the resultant
signal is outputted to the phase comparator 82 as a reference
signal to be used as a reference for correction.
[0055] The phase comparator 82 receives a light source control
signal VconB generated from the mixing circuit 53, and compares the
waveform of the light source control signal VconB with that of the
above reference signal (i.e., the corrected light source control
signal VoutA). The difference in ratio of light-on periods in one
vertical period is detected as a phase difference, and an error
component indicating the light-on period difference between the
light source control signal and the reference signal is outputted
to the signal processing unit 83.
[0056] The signal processing unit 83 generates, based on the
received error component, a correction signal that is to be used
for correcting the light source control signal so that the ratio of
light-on periods in one vertical period coincides with that
provided by the reference signal. The correction signal is
outputted to the phase comparator 82 described above.
[0057] The phase comparator 82 superimposes the correction signal
on the light source control signal VconB, so that the ratio of
light-on periods in one vertical period, provided by the light
source control signal VconB, coincides with that provided by the
reference signal. Thus, the phase comparator 82 generates a
corrected light source control signal VoutB capable of providing
the ratio of light-on periods in one vertical period, which
coincides with the ratio of light-on periods in one vertical
period, provided by the reference signal.
[0058] The corrected light source control signal VoutB is outputted
to the inverter unit 40 via the signal processing unit 83, and is
also outputted to the reference signal generating unit 81 of the
PLL 80. The reference signal generating unit 81 synchronizes the
corrected light source control signal VoutB with a vertical
synchronizing signal generated from the video signal processing
circuit (not shown), and outputs the resultant signal to the phase
comparator 82 as a reference signal to be used as a reference for
correction.
[0059] The phase comparator 82 receives a subsequent light source
control signal VconC, and compares the waveform of the signal with
that of the reference signal, i.e., just-received corrected light
source control signal VoutB that has been previously corrected as
described above. The difference in ratio of light-on periods in one
vertical period is detected as a phase difference. Thereafter,
correction is performed by the phase comparator 82 and the signal
processing unit 83 in a similar manner to the above correction, and
thereby a corrected light source control signal VoutC is generated.
The corrected light source control signal VoutC is outputted to the
inverter unit 40 and also to the reference signal generating unit
81, via the signal processing unit 83.
[0060] As described above, according to the present preferred
embodiment, the PLL 80 corrects a subsequently-received light
source control signal VconB so as to correct the ratio of light-on
periods in one vertical period, by using, as a reference, a
corrected light source control signal VoutA obtained by correcting
a previously-received light source control signal VconA. As a
result, a corrected light source control signal VoutB is generated.
Further, a newly-received light source control signal VconC is
corrected by using the corrected light source control signal VoutB
as a reference, and thereby a corrected light source control signal
VoutC is generated. Thus, the PLL 80 iterates the correction
cycles.
[0061] According to this construction, a corrected light source
control signal Vout as a previously-corrected signal capable of
constantly providing a steady light-on ratio is used as a reference
for correction. The signal corrected using such a reference signal
can also provide a constant light-on ratio. Therefore, when the
light sources are controlled according to the corrected light
source control signal Vout, the periods of light-on of light
sources, in one vertical period as a reference period for one frame
of images to be displayed, can be constant in time length,
regardless of respective pulse shapes of the light source control
signal Vcon. Consequently, the light sources can provide stable
brightness, which enables provision of a high-quality display
device.
Other Preferred Embodiments
[0062] Described above are preferred embodiments of the present
invention. However, the present invention is not limited to the
preferred embodiments explained in the above description with
reference to the drawings. The following preferred embodiments may
be included in the technical scope of the present invention, for
example.
[0063] In the above preferred embodiments, an aperiodic light
source control signal Vcon is generated preferably by performing a
logical operation on signals generated from the PWM signal
generating circuit 51 and the current-controlled dimming signal
generating circuit 52. However, the PWM signal generating circuit
need not necessarily be provided. That is, the aperiodic signal
generating circuit may directly generate alight source control
signal. In this case, an additional circuit is preferably provided
to synchronize the light source control signal with a vertical
synchronizing signal.
[0064] In the above preferred embodiment, two signals are
preferably randomly selected for each time period from signals
generated by three PWM signal generating circuits 71A, 71B, 71C.
However, the configuration of the present invention is not limited
to this combination, but rather may be any combination that
includes at least three PWM signal generating circuits so that at
least two PWM signals can be selected therefrom.
[0065] In the above preferred embodiments, a PWM signal or alight
source control signal VconA is used as a reference signal. However,
the reference signal is not limited to these signals. The reference
signal generating unit may be configured to output reference
signals having been prepared specially and preliminarily.
[0066] In the above preferred embodiments, cold cathode tubes 17
are preferably used as light sources, for example. However, another
type of light sources such as hot cathode tubes may be used, for
example.
[0067] In the above preferred embodiments, TFTs are preferably used
as switching elements of the liquid crystal display device, for
example. However, the present invention can be applied to a liquid
crystal display device that uses another type of switching element
other than TFTs (e.g., thin-film diodes (TFDs)). Further, the
present invention can be applied to a liquid crystal display device
for monochrome display, as well as a liquid crystal display device
capable of color display.
[0068] Moreover, although a liquid crystal display device is shown
in the above preferred embodiments, the present invention can be
applied to other types of display devices than a liquid crystal
type, which use a backlight device.
[0069] While preferred embodiments of the present invention have
been described above, it is to be understood that variations and
modifications will be apparent to those skilled in the art without
departing the scope and spirit of the present invention. The scope
of the present invention, therefore, is to be determined solely by
the following claims.
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